I'm blonde, so I like to simplify things as much as possible. I feel like I don't really have a firm grasp on this subject so here are my thoughts which I urge anyone to challenge in the hope it will help me better understand
The OSI layer, we've all heard of it? 7 odd layers. Here's a freshly stolen image from Google:
With simplification in mind, ignore everything bar layer 2/3/4 because it more or less doesn't really exist:
Also referred to as:
Natively, referred to as (the PDU/protocol data unit) of each layer:
The upper layers fit
inside the lower layers. They are 'encapsulated' by the lower layers. Imagine your foot, in a sock, in a shoe:
Thus, layer 2 is the largest layer because everything else is ultimately inside it.
Every layer has, more or less, the same format.
Data + Header. Data is whatever is being sent of any value and the headers contain things like where it came from and where it's going.
The Data+Header of layer 4, is stuffed into the Data portion of Layer 3
The Data+Header of layer 3, is stuffed into the Data portion of Layer 2
The 'Data' of layer 2, the frame, is effectively the gold. It is the MTU.
It looks a bit like this:
To further complicate things, some vendors will describe their MTU as the Data portion only. Some vendors will describe it as the Data+Header. Cisco and Juniper used to do this apparently, dunno whether it's still the case.
So what's the problem? PPPoE.
Our xDSL connections don't use ethernet from end to end. PPPoE is wedged in between. I don't really know why it's still used but I suspect for it's accounting features and because historically, it's always been used for xDSL/ATM.
So what layer does PPPoE sit on? The clue is in the name. PPP over Ethernet. 'Over' in this case means 'inside'. It sits inside that Data portion.
OK, cool. So it sits on layer 3?
That would be my natural conclusion too but it seems not. This is where the OSI gets a bit messy. PPPoE needs it's own layer because, it too, has a Data+Header construct.
8 bytes get robbed from ethernet to pay for it. Our Data portion is now squeezed to 1492 instead of 1500.
Oh, that's OK I guess? Fraid not!
As we said earlier, layer 2 is the largest layer because everything else fits inside it. If you nick 8 bytes from here, you nick 8 bytes from every layer above it
Here's the real problem. Your PC doesn't know you have a DSL modem limiting you to 1492 byte MTU. When you initiate a connection somewhere, via TCP (layer 4) the very first part of that 'handshake' goes a little like this:
<YourPC> Yo, Webserver, I'd like some content, plz? By the way, I just checked my layer 2 MTU and it appears to be 1500. Thus, you should send chunks of data to me based on that.
<Webserver> Hi PC. Sure thing, bawss. I just checked my own layer 2 MTU and funnily enough, mine is set to 1500 too.
<YourPC> Tidy! We appear to be in agreement. I will start requesting stuff shortly!
Every layer has it's own MTU. It's the Data portion. TCP calls it's version of MTU the MSS; the maximum segment size. I believe this is layer 2 MTU minus the layer 3 header and then minus the layer 4 header. Make sense? Based on a standard layer 2 MTU of 1500, MSS is 1460. Your PC and the webserver just negotiated that connection based on their local MTU settings. They are both completely unaware that you are stunted by a 1492 MTU along the path, caused by PPPoE.
When your ISP router receives a frame with a packet that is too large to send back to you, it is forced to shave off the excess and dump it into a second frame. This is called fragmentation. This is a pain in the ass. ISPs do not like doing this. They like using their very fast and expensive router ASICs to deal with simple forwarding decisions. Fragmentation, especially when it's happening for tens of thousands of subcribers, is a big problem that causes a large increase in router resource. It's also an unnecessary problem, if your underlying network supports larger frame sizes and lo and behold, PPPoE now has support for negotiating larger frame size to go with it.
So, whilst I can see why some ISPs are extolling the virtues of PPP-Max-Payload I think we should bear in mind, that although there are benefits for the end user, as above, the real benefits are for the ISP with a well setup and monitored network.
What makes me uneasy about this whole thing is things like this:
http://www.sinet.bt.com/sinet/sins/pdf/472v2p6.pdf
3.5.1 PPPoE Aspects
If the WBC Customer intends their End User’s to use PPPoE, they must ensure that the End Users
are provided with a PPPoE client compliant to RFC2516[15] and RFC1661[9].
The amended RFC that includes the larger payload support is RFC 4638. This makes me question whether this is properly supported on any of the BT BRAS/LAC servers?
Personally, I would not be touching this with a barge pole but I'll keep an open mind if anyone has info to the contrary.
If anything, I would be reducing my LAN MTU to hopefully escape all these issues
